In the automatic testing of electrical circuits, test probes of various configurations are used, depending upon such factors as the type of electrical device under test, the spacing between test points, and the like. One type of prior art test probe includes a movable plunger mounted in a tubular barrel with the probe end of the plunger extending outwardly from the barrel. A spring in the barrel supports the plunger for spring biased axial travel within the barrel. The plunger is biased outwardly a selected distance by the spring and may be biased inwardly into the barrel a selected distance under force directed against the spring. The probe plunger has a probe tip designed with various configurations for making contact with selected test points on the electrical device under test.
There are many applications in which probe end of the plunger may experience difficulty in making reliable contact with a particular circuit element or test point during the testing cycle. In order to ensure good contact during testing, prior art test probes have used certain probe tip configurations designed to enhance frictional contact when spring pressure is applied to the probe during testing. However, such an approach is not consistently the answer to the problem of making reliable contact.
Examples of instances in which good frictional contact between the test probe and the test point is desirable are where the board under test has been in a dirty environment, or where no de-fluxing was conducted after soldering. When circuit components are soldered to a board, free particles such as highly corrosive fluxes, chemicals, or other contaminants can be left on the board. Solvents have been used in the past for de-fluxing circuit boards, but solvents are being used less frequently today for environmental reasons. Therefore, many currently used circuit boards are not thoroughly cleaned before testing. In addition, spots formed on the board by the soldering process project from the board and are often covered with fluxes or oxide films. Standard test probes can have difficulty penetrating these areas of the board, producing unreliable test results.
To confront these problems, circuit boards are often tested not by standard test probes, but by probes in which the plunger is twisted about its axis during testing. This is intended to ensure that the probe tip will reliably penetrate the debris or film left on the board and make good electrical contact with the test point. In these types of test probes, the probe tip functions as a drill bit, turning on its axis as it presses against the board to cut through the debris or film that may be left on the board.
In addition to its penetrating function, such twisting test probes also must overcome additional problems. The testing of boards having free particles or debris often results in the probe contacting irregular surfaces on the board. This can produce undesired side loads on the probe plunger. A standard test probe may be able to withstand such side loading, but the process of making a twisted plunger reduces some of the plunger's effective cross-sectional area, which can reduce its ability to withstand side loads without undesired bending or flexing. Increasing the diameter of the twisted plunger section can improve the plunger's ability to withstand side loads, but increasing the diameter of the plunger is a disadvantage when test probes of smaller and smaller diameters are required for more diverse testing applications involving high density test patterns.
The demand for greater probe "pointing accuracy" also is becoming more important. There is an increasing need for the probe tip to reliably make point contact with test pads of increasingly smaller diameters within very small tolerance limits. Pointing accuracy becomes more important with more densely packed test patterns. If one or more of the probes does not align precisely with its target when the board is tested, the test results may be incorrect. Incorrectly aligned or pointed probes can miss the test point altogether. Therefore, the strike point of the probe used for testing circuit boards needs to be accurate within extremely close tolerances. This can be more of a problem for twisting test probes which, if not designed properly, can have a slight amount of wobble as the plunger twists about its axis when reciprocating in the barrel of the probe assembly.
Thus, there is a need for a twisting electrical contact test probe that can reliably penetrate debris or other contaminants on a circuit board under test, while resisting normal side loading during use in such an environment, and also while providing precisely controlled pointing accuracy.
A prior art twisting electrical test probe is disclosed in Japanese patent publication no. 60(1985)-127466, published Jul. 8, 1985. Another twisting electrical test probe having similar features is disclosed in U.S. Pat. No. 5,032,787 to Johnston, et al., assigned to the assignee of this application. These references disclose test probes in which the plunger has a twisted section with helical grooves engaged by crimps formed in the barrel between the open end of the barrel and the spring seating end of the barrel. The twisted portion of the plunger extends from inside the barrel where it contacts the crimps to the outside of the barrel where the tip of the plunger makes contact with the unit under test. An axial force directed at the plunger causes the crimps on the barrel to engage the helical grooves and twist the plunger about its axis as the plunger reciprocates in the barrel. These twisting electrical test probes can have several disadvantages. One has to do with manufacturing problems. If the crimps are too tight, the plunger can stick in the barrel, and if the crimps are too loose, the probe tip can have a tendency to wobble. Exposed twisted end sections of the plunger, if reduced in diameter to small thicknesses necessary for certain high density testing, can be too long and thin, leading to structural weakness that makes these probes impractical for high density testing.
The present invention overcomes these problems by providing a twisting electrical test probe that produces smooth operation and is well constrained, avoiding the tendency to wobble and providing high pointing accuracy.
U.S. Pat. No. 5,009,613 to Langgard, et al. discloses a twisting electrical test probe in which the plunger includes a probe tip section projecting from the open end of the barrel, a spring seating end within the barrel biased by the spring, and an intermediate twisted section with helical grooves between the spring seating end of the probe tip section of the plunger. In the probe disclosed in the '613 patent, the twisted section is contained internally within the barrel and is engaged by crimps formed in a intermediate wall portion of the barrel. The portion of the barrel between the crimps and open end on the barrel is slidably engaged by a cylindrical shaft spaced inboard from the probe tip for closing off the open end of the barrel. The sliding shaft near the open end of the barrel is said to prevent contamination from debris entering the barrel and to improve pointing accuracy.
At the probe end of the plunger, pointing accuracy is affected by a tolerance build-up in the probe assembly. In the Langgard, et al. '613 probe, there are three tolerances that affect pointing accuracy: (1) the tolerance between the barrel in the receptacle, (2) the tolerance between the receptacle and the hole in which it is mounted, and (3) the tolerance between the plunger and the barrel. These combined tolerances can lead to a tolerance build-up that can adversely affect pointing accuracy at the probe end of the plunger.
The present invention provides a twisting electrical test probe that twists about its axis to penetrate debris and other contaminants while testing printed circuit boards and the like. In addition to providing improved pointing accuracy, the probe has a controlling plunger diameter that is enlarged sufficiently to significantly increase the strength of the probe to better withstand side load forces. The probe improves upon the pointing accuracy of the prior art Japanese '466 and Johnston, et al. '787 probes described above, while also providing improved probe strength to better resist side loads when compared with the Langgard, et al. 613 probe. The probe also has a better resistance to tolerance build-up that affects pointing accuracy, when compared with the Langgard, et al. '613 probe. Tolerances affecting pointing accuracy are limited the tolerance between the receptacle and the hole in which it is mounted and the tolerance between the plunger and the receptacle where the plunger is guided by the receptacle. These improvements will be better understood with reference to the detailed description to follow.